Vacuum chucks are commonly used to hold wafers. For example, vacuum chucks can be used to hold semiconductor wafers during inspection or during other periods of wafer manufacturing. Vacuum chucks typically have a chucking surface that contacts the wafer. One or more vacuum grooves extend through this chucking surface. Suction forces retain a wafer on the vacuum chuck when air or another gas is evacuated through the vacuum groove or grooves. A pressure difference between the chucking surface and opposing wafer surface holds the wafer in place during processing or can flatten the wafer against the vacuum chuck.
Not all wafers are perfectly planar. Wafers can become warped. This renders the surface of the wafer curved or otherwise irregular (i.e., non-planar). For example, a point on the surface of a warped wafer can deviate from a reference plane relative to a circumference of the surface. Such warpage may be a result of wafer processing or a result of stress or strain to the wafer. For example, layers or films on the wafer can cause stress or strain that leads to warping.
Sufficiently securing a warped wafer to a vacuum chuck can be difficult. The flatness of the wafer surface affects how well a vacuum chuck can secure a wafer. A vacuum chuck is less effective at securing or flattening a wafer as the distance between the vacuum chuck and the wafer surface increases. Thus, if part of the wafer surface is farther away from the vacuum chuck due to warpage, this part of the wafer surface may not be secured or flattened using the vacuum chuck. An unsecured wafer or wafer that is not properly flattened can affect wafer manufacturing.
In semiconductor applications using metrology systems, some wafers are warped to such a degree that these warped wafers cannot normally be processed or handled by an inspection system. Such wafers may be warped as a result of upstream manufacturing processes (e.g., metal deposition) that change the planar flatness and cause these wafers to be bowl-shaped, dome-shaped, or asymmetrical (twisted). Metrology tools, such as bright field tools, often require substantially flat wafers. Wafers are typically clamped with a chuck to flatten them for metrology or processing. Warped wafers often cannot be clamped on a vacuum chuck to flatten them sufficiently for inspection in a metrology system. Consequently, a significant loss of yield through the metrology system can result from the warping of the wafers.
Vacuum chucks typically can secure a wafer with warpage of up to 0.1 mm. In spite of the 0.1 mm warpage, the wafer can be pulled flat against the chucking surface and sufficiently retained using just the suction force of a vacuum chuck. Chucking of warped wafers with warpage from 0.4 mm to 0.8 mm on a vacuum chuck may be possible in some instances, but this may be limited to particular warpage orientations or wafers with specific backside surface finishes. However, some wafers have warpage greater than 0.8 mm. For example, some wafers have warpage up to approximately 5 mm or 6 mm. Wafers that are warped to such a degree cannot be sufficiently secured to existing vacuum chucks. Therefore, what is needed is a new apparatus and method to chuck warped wafers.